Now, Japan faces an aging society, which involves increasing cost of medical care. Accordingly, sound management of medical insurance system becomes increasingly difficult. These and other related problems draw growing interest in health management and disease prevention in the society. What is desired is a society focusing more heavily on preventive care (here, such a society will be referred to as “preventive-care-oriented society”) than cures after development of diseases.
To attain such an object, a system that allows handy and speedy checking of one's health condition is necessary. Biological information that can easily be sampled for grasping one's health condition includes blood, urine, sweat, saliva and expired gas. Among various components of these, a certain substance generates or its volume changes as a sign of potential disease or as a result of some disease that one suffers from, and by knowing variation thereof, it is highly possible to grasp one's health condition. Such a substance is called a marker. Marker monitoring helpfully leads to early detection and rapid cure.
Non-Patent Document 1 discusses relations between diseases and markers. Table 1 shows part of the relations.
TABLE 1BiologicalDiseaseInformationLarge ClassificationSmall ClassificationMarkerExpired gasOxidant stressLipid oxidation,Ethane,asthma, bronchialpentane,infectionH2O2Pulmonary diseaseAsthma, chronicNO, CO, H2O2obstructivepneumoniaGastrointestinalIndigestion,H2, carbondiseasegastritis,isotopeduodenal ulcerMetabolicDiabetesAcetoneabnormalityKidney diseaseAmmoniaPeriodental diseaseMercaptanSaliva, UrineStressGlobulin A,cortisol,amylase,chromograninA, immuneglobulinSweatFatigueAmmoniaTearFatigueACTH
Of the biological information, expired gas exists close to the blood in capillary vessel separated only by a thin membrane in the lung and, therefore, it particularly contains many markers. Further, its sampling is easy. Therefore, it is the most useful biological information to be monitored.
As a method of monitoring these markers in the expired gas, a gas sensing element may be available. Conventionally, an oxide semiconductor gas sensor using tin oxide has been known. The lower detection limit of sensor sensitivity is at the level of down to 103 ppm, and for monitoring, it is necessary to heat the sample to about 300° C. As regards the monitoring of expired gas, according to Non-Patent Document 1, high sensitivity with lower detection limit at the level of ppb is necessary. Further, the expired gas is highly prone to change when heated, and non-invasive method is desired. Therefore, for this purpose, an oxide semiconductor gas sensor is inappropriate. To satisfy social demand, higher performance is necessary.
As a solution to such a problem, Non-Patent Document 2, which will be described later, proposes a gas sensor formed of recently discovered carbon nano tube (hereinafter denoted as “CNT”).
CNT is a tubular carbon material whose diameter is on the nano order. A crystal structure having a plate-shaped structure of regular hexagons bonded together, each consisting of six carbon atoms, is called a graphite structure. The graphite structure continued two-dimensionally is referred to as a graphane sheet. CNT has a structure formed by rolling the graphane sheet to a cylinder. CNT has a very stable structure and exhibits electric conductivity allowing high-speed electron migration derived from Π electron coupling therein, and it is known to be a conductor superior to metal wires depending on the structure.
According to Non-Patent Document 2, when chemical substance molecules adhere to CNT, electron migration occurs, and electromotive force generates. In other words, between two points on CNT, difference in potentials or change in electric resistance occurs. By detecting the change in electric resistance, the chemical substance can be sensed. Further, since CNT has a miniature structure on the nano order, significant improvement can be expected in responsiveness and detection lower limit. Specifically, the time from when the specific chemical substance adheres to the CNT surface until electric resistance of CNT changes is very short because of the conductivity and nanostructure of CNT, as compared with the conventional sensing element described above. Further, since CNT has large surface area and has such a structure that every atom constitutes the surface, the influence of substance adsorption is reflected on the change in electric resistance with only a little loss resulting from electron scattering and the like. Therefore, CNT is believed to enable adsorption and confirmation of presence of specific chemical substance of a small amount, which have been difficult by a conventional sensing element.
Further, because of the nanostructure of CNT, the chemical substance sensing element using the CNT realizes a very compact, low-power consumption and portable element, which is optimal for a simple means for checking individual health condition.
As a general sensor using CNT, one such as described in Patent Document 1 has been known. This reference, however, does not include any description related to monitoring of markers included in biological information as described above, and details of sensor sensitivity are not described, either.    Non-Patent Document 1: Wenqing Cao et al., “Breath Analysis: Potential for Clinical Diagnosis and Exposure Assessment,” Clinical Chemistry, vol. 52. 5, p. 800-811, 2006    Non-Patent Document 2: Ri'ichiro SAITO, “Outline and Problems of Carbon Nano Tube,” KINOU ZAIRYOU (Functional Material), vol. 21, No. 5, p. 6-14, May, 2001    Patent Document 1: Japanese Patent National Publication No. 2003-517604